A transconductor may be a transconductance amplifier that generates a change in output current with a change in input voltage. An actual transconductance amplifier, such as in a mixer of an RF receiver, produces some distortion, such as generating second and third harmonics of the fundamental frequency, generating frequency-mixed signals, and generating intermodulation products. For example, a typical transconductance amplifier receiving sine waves f1 and f2 will output the following signals, having various magnitudes. The second and third order signals are output due to distortion:
TERMOUTPUTFREQUENCYlinearfundamentalf1, f22nd order2nd harmonic2f1, 2f23rd order3rd harmonic3f1, 3f22nd orderfrequency mixing(f2 − f1), (f2 + f1)3rd order3rd order intermod. products(2f2 − f1), (2f1 − f2)
Second and third order signals may also be generated by RF interference.
The output current of a non-ideal transconductance amplifier can be described by the following power series, limited to the third order, where transconductance (represented by the coefficient an) is defined as the change in output current (i(t)) with a change in input voltage (v(t)):i(t)=a0+a1v(t)+a2v(t)2+a3v(t)3+ . . . ,  Eq.1where the dc quiescent current is represented by a0, the linear transconductance is represented by a1, the second-order non-linearity is represented by a2, and the third-order non-linearity is represented by a3. The third order intermodulation (IM3) products are a3v(t)3. Transconductance is also referred to herein as gm.
The IM3 products are the most problematic in some situations since they may occur near a fundamental frequency and may be difficult to filter out.
Transconductance amplifiers are typically used in mixers forming part of a demodulator of an RF receiver.
There are many types of prior art IM3 cancellers. Some predistort the input voltage to compensate for the IM3 distortion. Some require differential signals to generate an IM3 correction. RF signals in the gigahertz range may be difficult to accurately convert to differential signals and may require transformers that add expense and real estate. Various drawbacks exist with the prior art IM3 cancellers, including imprecision due to process and temperature variations, the difficulty in creating differential signals at high frequencies due to component limitations, complexity, real estate requirements, and other issues.
What is needed is a simpler IM3 canceller that can operate at high frequencies, such as at gigahertz frequencies for cell phones, and is self-adjusting for process and temperature variations.